Heat StressThe Impact of Acute Exercise in the Heat on Breast Milk Production and Composition in Lactating Women

This clinical trial aims to determine whether heat stress alters 24-hour breast milk production and composition following an acute bout of exercise in the heat among lactating women.

The main questions are:

1. To ascertain whether heat stress alters 24-hour breast milk production following acute exercise in the heat among lactating women. We hypothesize that breast milk production will decrease to a greater extent following acute exercise in the heat compared to acute exercise with no heat stress. Lactating mothers will complete an acute exercise bout at 40% of their peak oxygen consumption (VO2peak) for 60 mins in a hot (36 °C, 40% relative humidity) and thermoneutral environment (20 °C, 20% relative humidity). Participants will record their total breast milk production in a 24-hour period using an infant scale.
2. to discover whether heat stress impacts the energy composition of breast milk following acute exercise in the heat among lactating women. We hypothesize that energy density will decrease to a greater extent following acute exercise in the heat compared to acute exercise with no heat stress. Participants will use a manual expression breast pump to collect 10 mL of breast milk from each breast immediately and 24 hours post-exercise. Energy density (lactose, protein, and lipid content) will be assessed via enzyme-linked immunosorbent assays.

Participants will complete the following visits: 1) initial consent visit 2) baseline testing; 3) acute exercise in the hot condition (HOT; 36C, 40% relative humidity), and 4) acute exercise in the temperate, thermoneutral condition (TEMP; 20C, 20% relative humidity). The baseline testing day will measure participant anthropometrics including height, weight, and body composition measured by a DEXA scan. For both experimental trials, participants will be asked to walk on a treadmill at based on their metabolic heat production (8W/kg) for 60 minutes. Heart rate (HR), mean skin temperature (Tsk), core temperature (Tcore), sweat loss (SL), and fluid intake will be measured throughout the exercise protocol. Breast milk composition will be measured through milk expression using a manual expression breast pump immediately and 24 hours post-exercise. Participants will log their breast milk production for 24 hours following the exercise using an infant scale to measure their infants before and after each feed.